Photocatalytic Partial Water Dissociation by Protonated Carbon Nitride for Hydrogenation Reactions

Catalytic hydrogenation using pressurized hydrogen at elevated temperature is of utmost importance in refinery and fine chemical industries, but heavily relies on the hydrocarbon reforming industry. Photocatalytic water dissociation provides a sustainable solution for hydrogenation; however, the strong O─H bond and instant recombination of generated hydrogen atoms and hydroxyl radicals need to be solved. Here, we demonstrate efficient hydrogenation of unsaturated aromatics using water by a protonated graphitic carbon nitride photocatalyst with dioxane as a hydroxyl radical trap under mild conditions. The dissociation energy of the H─O bond is reduced from 5.15 to only 1.48 eV, enabling the formation of H atoms and surface adsorbed hydroxyl radicals (^•OH_ads) under visible light. The ^•OH_ads reacts with dioxane, yielding value-added dioxane-2-ol and prolonging the lifetime of hydrogen atoms for hydrogenation. A high quantum efficiency of ∼6% can be realized under visible light irradiation for the selective hydrogenation of aromatic bromides and aldehydes with high conversion. This approach is scalable in a flow system, revealing financial and environmental potential for hydrogenation and deuteration at a practical level.